Hydro-pneumatic tensioner with stiffness altering secondary accumulator

ABSTRACT

A hydro-pneumatic tensioner includes a barrel having an inner bore and a pressurized fluid contained within to form at least part of a primary accumulator having a preset volume of gas at a preselected pressure. A piston having a piston rod extending from an aperture in the barrel is slidably carried in the bore of the barrel and is in communication with the pressurized fluid and positioned to increase the fluid pressure when the piston strokes in the direction of the pressurized fluid. A secondary accumulator also has a preset volume of gas at a preselected pressure. A fluid separator maintains functional separation of the fluid volumes of the primary and secondary accumulators when the primary accumulator pressure is less than a preselected secondary accumulator pressure. The fluid separator allows functional combining of the fluid volumes of the primary and secondary accumulators when the primary accumulator pressure equals or is greater than the preselected secondary accumulator pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to tensioning of seabed-to-vessel marine risers.More particularly, this invention relates to tensioning the marinerisers with a plurality of pneumatic or hydraulic cylinders.

2. Brief Description of the Related Art

A problem presented by offshore hydrocarbon drilling and producingoperations conducted from a floating platform is the need to establish asealed fluid pathway between each borehole or well at the ocean floorand the work deck of the platform at the ocean surface. A risertypically provides this sealed fluid pathway. In drilling operations,the drill string extends through a drilling riser, the drilling riserserving to protect the drill string and to provide a return pathwayoutside the drill string for drilling fluids. In producing operations, aproduction riser is used to provide a pathway for the transmission ofoil and gas to the work deck.

The riser is projected up through an opening referred to as a moon poolin the vessel to working equipment and connections proximate anoperational floor on the vessel. A riser pipe operating on the floatingvessel in water depths greater than about 200 feet (34.72 meters) canbuckle under the influence of its own weight and the weight of drillingfluid contained within the riser if it is not partially or completelysupported. For floating platforms, a special piece of equipment known asa “riser tensioner” is required to maintain each riser within a range ofsafe operating tensions as the work deck moves relative to the upperportion of the riser. If a portion of the riser is permitted to go intocompression, it could be damaged by buckling or by bending and collidingwith adjacent risers. It is also necessary to ensure that the riser isnot over-tensioned when the vessel hull moves to an extreme lateralposition under extreme wave conditions or when ocean currents exert asignificant side loading on the riser.

A tension leg platform (“TLP”) is a type of marine structure having abuoyant hull secured to a foundation on the ocean floor by a set oftethers. The tethers are each attached to the buoyant hull so that thehull is maintained at a significantly greater draft than it would assumeif free floating. The resultant buoyant force of the hull exerts anupward loading on the tethers, maintaining them in tension. Thetensioned tethers limit vertical motion of the hull, thus substantiallyrestraining it from pitch, roll and heave motions induced by waves,currents and wind. However, unlike conventional platforms which arerigidly attached to the subsea floor, TLPs are not designed to resisthorizontal forces induced by waves.

The marine risers have been tensioned in various manners, including theuse of counterweights and pneumatic spring systems. The counterweightwas the first technique utilized to apply tension to the top of themarine riser. The weight was hung from a wire rope which was run upthrough wire rope sheaves and down to an upper portion of the riserpipe. The tension was equal to the counterweight and therefore waspracticable only for shallow water that required low tension.

The pneumatic spring systems replaced the counterweight systems asdeeper water drilling evolved. The pneumatic tensioning devices storedenergy in the form of compressed air to apply tension to the top of theriser through wire ropes. The pneumatic tensioning devices typicallyinvolved the use of cylinders from which a piston rod was extended, thepiston rod having a sheave engaged by the wire rope to be tensioned. Thefluid within the hydraulic cylinder was thereby compressed into anaccumulator. The cylinder and the accumulator were normally supported bysupport structures on the floating vessel.

Many tensioner systems in use today act as oil-damped pneumatic springs.A large gas supply keeps a nearly constant pressure above the oil in agas-oil accumulator. The oil then provides pressure to the face of thepiston. As the vessel heaves, the piston moves up and down against arelatively constant force and the tension lines maintain a relativelyconstant pull.

Many riser tensioners today utilize hydraulically actuated cylinderswith pneumatic pressure accumulators to provide the force necessary tomaintain the upper portion of the riser within a preselected range ofoperating tensions. One implementation is accomplished by the use ofsheaves attached to the buoyant drilling structure whereby tensioningcables are run over the sheaves and attached to the riser so that theriser is supported by one end of the tensioning cables. The other end ofeach tensioning cable is connected to a piston of a hydraulic cylinder.The hydraulic cylinders are connected to a relatively large accumulatorwhich maintains the load applied by the cylinders at a relativelyconstant level over the full range of travel of the pistons. Thus, asthe platform moves vertically, the pistons stroke to maintain arelatively constant upward loading on the riser.

Another type of riser tensioner typically used on TLPs also uses apneumatically pressurized fluid accumulator but eliminates the cablesand sheaves used in earlier riser tensioners. Gas and oil accumulatorsare connected to the cylinders to control the stroke of pistons. Thepiston rods are directly attached to a riser tensioning ring whichsupports the riser.

Both classes of riser tensioning systems described generally requireseparate and relatively large accumulators to maintain the load appliedby the cylinders with an acceptable range. Accordingly, it can beappreciated that there still exists a need for an improved risertensioner system which provides high-capacity tension and provides forlimiting peak loads while incorporating high nominal stiffness, and thatdoes not require an excessively large accumulator.

SUMMARY OF THE INVENTION

In view of the foregoing, an embodiment of the present inventionadvantageously provides a tensioner unit including a barrel having abore with pressurized fluid contained within. The barrel forms at leastpart of a primary accumulator having a preset volume of gas Vg1 atpressure P1. The barrel of the tensioner unit also includes an apertureto allow extension and retraction of a piston rod. The tensioner unitalso includes a piston slidably carried in the bore of the barrel. Thepiston has a piston rod that extends from one side of the piston andthrough an aperture in the barrel. The piston has one of its sides incommunication with the pressurized fluid, and is so positioned toincrease the pressure P1 of the primary accumulator when the pistonstrokes in the direction of the pressurized fluid. The tensioner unitalso includes a secondary accumulator housing having a bore which formsat least part of a secondary accumulator having a preset volume of gasVg2 at pressure P2. The tensioner unit also includes a fluid separatorpositioned between the primary and secondary accumulators to maintainfunctional separation of fluid volumes of the primary and secondaryaccumulators when the primary accumulator pressure P1 is less than thesecondary accumulator pressure P2. The fluid separator also allowsfunctional combining of the fluid volumes of the primary and secondaryaccumulators when the primary accumulator pressure P1 is greater than orequal to the secondary accumulator pressure P2. The tensioner unit isconfigured so that when the primary accumulator pressure P1 is less thanthe secondary accumulator pressure P2, the effective total gas volumeVgT available to the tensioner to maintain tension on a supportedsystem, such as a riser system, is substantially equivalent to theprimary accumulator gas volume Vg1. Correspondingly, when the pressureP1 is greater than or equal to the secondary accumulator pressure P2,the effective total gas volume VgT available to the tensioner tomaintain tension on the supported system equals the sum of the primaryaccumulator gas volume Vg1 plus the secondary accumulator gas volumeVg2. This provides for reduced stiffness and reduced maximum tensionapplied by the tensioner unit to a supported system when the system isdirecting a maximum load on the tensioner unit.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features, advantages and objects of theinvention, as well as others which will become apparent, are attainedand can be understood in more detail, more particular description of theinvention briefly summarized above may be had by reference to theembodiment thereof which is illustrated in the appended drawings, whichdrawings form a part of this specification. It is to be noted, however,that the drawings illustrate only a preferred embodiment of theinvention and is therefore not to be considered limiting of its scope asthe invention may admit to other equally effective embodiments.

FIG. 1 is a perspective view of a hydro-pneumatic tensioner system inaccordance with the invention.

FIG. 2 is an enlarged perspective view of one of the hydro-pneumatictensioner units of the hydro-pneumatic tensioner system of FIG. 1.

FIG. 3 is a schematic view of the hydro-pneumatic tensioner unit havinga first piston enclosing a second piston shown with the first piston inan extended position.

FIG. 4 is a schematic view of the hydro-pneumatic tensioner unit of FIG.3, shown a mid-point position.

FIG. 5 is a schematic view of the hydro-pneumatic tensioner unit of FIG.3, shown a retracted position.

FIG. 6 is a schematic view of an embodiment of a hydro-pneumatictensioner unit, including an internal cylinder barrel connected to anexternal cylinder having a second piston.

FIG. 7 is a schematic view of a variation of FIG. 6.

FIG. 8 is a schematic view of a variation of FIG. 6 having no internalcylinder barrel.

FIG. 9 is a schematic view of an external cylinder for use with thetensioner unit of Figure 6, having a bladder which forms an enclosurecontaining a gas volume Vg2 and a hydraulic volume Vh1.

FIG. 10 is a schematic view of an external cylinder for use with thetensioner unit of FIG. 6, having a pilot valve interface that forms anenclosure containing a gas volume Vg2 and a hydraulic volume Vh2.

FIG. 11 is a schematic view of another embodiment of a hydro-pneumatictensioner unit, having a first piston connected to an external cylinderhaving a second piston, shown with the first piston in an extendedposition.

FIG. 12 is a schematic view of the hydro-pneumatic tensioner unit ofFIG. 11, shown with the first piston in a mid-point position.

FIG. 13 is a schematic view of the hydro-pneumatic tensioner unit ofFIG. 11, shown with the first piston in a retracted position.

FIG. 14 is a schematic view of an external cylinder for use with thetensioner unit of FIG. 11, having a bladder which forms an enclosurecontaining a gas volume Vg2.

FIG. 15 is a schematic view of an external cylinder for use with thetensioner unit of FIG. 11, having a pilot valve interface that forms anenclosure containing a gas volume Vg2.

FIG. 16 is a schematic view of a variation of FIG. 7.

FIG. 17 is a schematic view of a pull-type hydro-pneumatic tensionerunit having a first external cylinder hydraulically connected to thehydraulic side of a first piston.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more fully hereinafter withreference to the accompanying drawings which illustrate embodiments ofthe invention. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theillustrated embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout, and the prime notation,if used, indicates similar elements in alternative embodiments.

Referring to FIG. 1, shown is a hydro-pneumatic riser tensioner system20 for maintaining a riser system within a range of safe operatingtensions as a work deck or platform moves relative to the upper portionof the riser system. The system 20 advantageously provides a methodologyfor incorporating high nominal stiffness while limiting peak loads atextreme up and down strokes. More specifically, the system 20 includes ariser system 22, extending between a floating vessel having anoperational platform 24, 26, and the sea bottom (not shown).

The riser system 22 is supported in tension at its upper end to preventthe riser system 22 from buckling due to its own weight. The upwardtension is maintained within acceptable limits over a typical platformdeviation range by the hydro-pneumatic riser tensioner system 20. Theriser tensioner system 20, discussed in detail below, includes aplurality of hydro-pneumatic tensioner units 28. The tensioner units 28include a main body or housing 30, typically tubular shaped,stationarily located between support frames 24, 26, of a support module23. Each tensioner unit 28 includes a hydro-pneumatic ram tensionerpiston rod 32 attached at its upper end to a riser tensional mount ortop plate 34, which includes a riser tensional ring 36. The risertensional ring 36 engages a tension joint 38 of the riser system 22. Thehydro-pneumatic ram tensioner piston rod 32 telescopes relative tohousing 30 in response to movement of the operational platform.

Referring now to FIGS. 1 and 2, in a typical implementation of thehydro-pneumatic tensioner system 20, typically each hydro-pneumatictensioner unit upper section is attached to the top plate 34 and thelower section is connected to the support module lower support frame 24,with each unit 28 protruding through an aperture 27 in the upper supportframe 26 of the support module 23. In this implementation, the risersystem 22 includes a riser conductor 42 and roller assemblies 44, onlower and upper support frames 24, 26, of the support module 23 toprovide for axial movement of the support module 23 with respect to theriser system 22.

In this embodiment, as the support module 23 shifts position withrespect to the riser system 22, the riser system 22 slides up and downthrough the roller assemblies 44 and is engaged with tension from thehydro-pneumatic riser tensioner system 20 through the tensioner units 28applying tension to the top plate 34. The top plate 34 via the mountingring 36 translates the tension from the tensioner units 28 to thetension joint 38 of the riser system 22.

Referring now to FIGS. 2 and 3, as stated, the hydro-pneumatic risertensioner system 20 includes a plurality of hydro-pneumatic tensionerunits 28. Each of the units 28 has an upper end and a lower end. Theupper end can include a rod end cap 46. The rod end cap 46, and thus theupper end of each unit 28 can be connected to the top plate 34 (FIG. 1)to provide the requisite tension to the tension ring 36 (FIG. 1) andthus to the riser system 22. The rod end cap 46 is shown as threadinglyattached to a shoulder or flange 48 formed of or attached to the mainbody of piston rod 32, using a plurality of bolts 50, each affixed in abolt cavity (not shown), however, any appropriate attachment or sealingmethodology as known by those skilled in the art is appropriate andwithin the scope of the invention. In an embodiment, the lower end ofthe tension unit 28 is connected to the operational marine platform (notshown). In another embodiment, the lower end of the units 28 isconnected to a lower platform or frame 24 of a hydro-pneumatic tensionersupport module 23 (FIG. 1).

Referring to FIG. 3 for illustrative purposes only, generic toembodiments, described below, the tensioner unit 28 includes a barrelsuch as inner cylinder barrel 60 or main body 30 each having a bore andan aperture on at least one end and having a pressurized fluid containedwithin. The barrel forms at least part of a primary accumulator having apreset volume of gas Vg1 at pressure P1. A piston such as piston 72 isslidably carried in the bore of the barrel. The piston has a piston rodthat extends from a side of the piston and through the aperture of thebarrel. One side 82 of the piston 72 is in communication with thepressurized fluid. The piston 72 is positioned to increase pressure P1of the primary accumulator when the piston 72 strokes in the directionof the pressurized fluid. A secondary accumulator housing, such as thepiston rod 32 with inner bore 76 or the external housings shown in FIG.9 or 12-15 (described later) form at least part of a secondaryaccumulator having a preset volume of gas Vg2 at pressure P2. A fluidseparator, such as floating piston 92 (FIG. 3), or the bladder or valvearrangement shown in FIGS. 9-12 (described later), is positioned betweenthe primary and secondary accumulators to maintain functional separationof fluid volumes of the primary and secondary accumulators when theprimary accumulator pressure P1 is less than the secondary accumulatorpressure P2, and to allow functional combining of the fluid volumes ofthe primary and secondary accumulators when the primary accumulatorpressure P1 is greater than or equal to the secondary accumulatorpressure P2. This arrangement results in an effective total gas volumeVgT available to the tensioner 28 to maintain tension on a supportedsystem, such as a riser system, to substantially equal the primaryaccumulator gas volume Vg1 when the primary accumulator pressure P1 isless than the secondary accumulator pressure P2, and to substantiallyequal the sum of the primary accumulator gas volume Vg1 plus thesecondary accumulator gas volume Vg2 when the pressure P1 is greaterthan or equal to the secondary accumulator pressure P2. The specificdetails of the various embodiments implementing the above described arefurther introduced below.

Referring to the embodiment shown in FIG. 3, tensioner unit 28 includesa primary accumulator 52. The primary accumulator 52 includes an upperend having a piston seal housing aperture 54, a lower end preferablyincluding an endcap 56 (FIG. 2), and a main body 30 having an axial bore58, therebetween. The combination, as described, along with internalcomponents, described below, form an enclosure. The primary accumulator52 contains a gas volume Vg1 and a hydraulic volume Vh1. Also in anembodiment, the lower end cap 56 (FIG. 2) is threadingly connectedwithin a lower end bore (not shown), although any appropriate attachmentor seal means as known by those skilled in the art is acceptable andwithin the scope of the invention. The sealed enclosure of the primaryaccumulator 52 forms a gas and hydraulic fluid reservoir. A chargingconnection (not shown) can be interfaced with the main body 30 of theaccumulator 52 to permit intermittent or continuous charging of both thegas and hydraulic volumes Vg1, Vh1, from an external pressure source(not shown).

Referring to FIGS. 3-5, the hydro-pneumatic riser tensioner system 20(FIG. 1) also can include a stationery or internal cylinder barrel 60located within the primary accumulator 52. The cylinder barrel 60 issecured sealingly to aperture 54 of accumulator 52. The cylinder barrel60 includes an upper end having an upper piston rod aperture 62, a lowerend having a lower fluid inlet aperture 64, and a main body having anaxial inner bore 66, therebetween. In the preferred embodiment, thecylinder barrel 60 includes a lower fluid aperture channel extension 68which protrudes to a position adjacent the lower end of primaryaccumulator 52, and provides for fluid communication between thecylindrical barrel inner bore 66 and primary accumulator hydraulicvolume Vh1. The length of aperture channel extension 68 is generallyformulated for the purpose of ensuring only hydraulic fluid entry intothe cylinder barrel inner bore 66, not gas. The length of the channelextension 68 may be established depending upon the operating conditionsthe unit 28 will be subjected to and the desired ratio of gas Vg1 tofluid Vh1. The combination, as described, along with internalcomponents, described below, form a partial enclosure. The cylinderbarrel 60 interfaces and is thus in fluid communication through itslower aperture 68 with the primary accumulator hydraulic volume Vh1. Thevolume of hydraulic fluid contained within the cylinder barrel 60 isdirectly dependent upon the gas pressure P1 within the primaryaccumulator 52 and stroke position of the tensioner piston rod 32.

The tensioner 28 has a first piston assembly 70 that moves axiallywithin barrel 60. The piston assembly 70 includes a combination firstpiston 72 and piston rod 32. The piston assembly 70 also includes anupper end typically interfaced with a rod end cap 46 (FIG. 2), a lowerend having a piston 72 including an aperture 74, and a main bodytherebetween including an inner bore 76. The combination, as describedabove and as to be described below, forms a channel which forms anenclosure of varying size depending on primary accumulator gas pressureP1 and stroke position of the riser tensioner piston rod 32. In thisembodiment, preferably the aperture 74 of piston 72 is not the samediameter as bore 76. The piston 72 has an outer diameter substantiallyequivalent to the inner diameter of the axial inner bore 66 of cylinderbarrel 60. Also, preferably, the outer diameter of piston rod 32 issubstantially equivalent to the diameter of piston rod aperture 62.

The first piston assembly 70 is slidably positioned to telescope frominside the cylinder barrel 60 to provide tension and stiffness to theriser system 22. Preferably the outer diameter of piston rod 32 issmaller than the inner diameter of cylinder barrel inner bore 66. Thisdifferential diameter between the first piston diameter/cylindricalbarrel inner bore diameter and the piston rod main body diameter/pistonrod aperture diameter forms a back annulus 78. The back annulus 78 ispreferably vented to the atmosphere or through vent lines (not shown)following an appropriate methodology as known by those skilled in theart.

The telescopic range of the first piston 72 can be limited by stops,described below, which present an upper and lower limit for the travelof the first piston 72. In the configuration shown in FIGS. 3-5, thedifferential diameter between the piston rod 32 and first piston 72 forma shoulder 79 on the upper surface of the first piston 72 adjacent itsouter circumference. The upper limit is the position where the firstpiston upper shoulder 79 contacts the lower surface 80 of the upper endof the cylinder barrel 60. The lower limit is a position where the lowersurface 82 of the first piston 72 contacts the upper surface 84 of thebottom of the cylinder barrel 60.

The piston assembly inner bore 76 interfaces and is in fluidcommunication through its lower aperture 64 with hydraulic fluid in thecylinder barrel 60, which as previously stated is in communication withprimary accumulator hydraulic volume Vh1. The volume of hydraulic fluidentering the piston assembly inner bore 76 adjacent its lower side isdirectly dependent upon gas pressure P1 within the primary accumulator52, which is directly dependent upon stroke position of the tensionerpiston rod 32. The first piston 72 includes seals 90 that slidablyengage and seal the bore 66 of barrel 60.

The first piston 72, particularly, the inner bore 76, encloses a secondpiston 92 which form a housing for a secondary accumulator. The secondpiston 92, a fluid separator, has an upper and lower surface 94, 96, anda diameter that is substantially equal to the diameter of the firstpiston rod inner bore 76. The second piston 92 is slidably carriedwithin the piston rod inner bore 76. In the embodiment shown, the secondpiston 92 includes pressure seals that slidingly engage inner bore 76.The second piston upper surface 94 in combination with the piston rodinner bore 76 form an enclosure, which along with a gas volume Vg2 andhydraulic volume Vh2, sealingly contained on the upper side 94 of thesecond piston 92, define a secondary accumulator 98 having a gaspressure P2. In a preferred variation, a charging connection (not shown)is located on the piston assembly 70 to permit intermittent orcontinuous charging of both the gas Vg2 and hydraulic volumes Vh2 froman external pressure source (not shown), depending on the embodimentimplemented. In an alternative variation, the secondary accumulator 98may include an interface (not shown) with an external accumulator (notshown) to provide added gas volume Vg2. Although FIGS. 3-5 show use ofhydraulic volumes Vh1 and Vh2, the primary accumulator 52 and secondaryaccumulator 98 can be made purely of a gas having volume Vg1 and Vg2,respectively. The use of hydraulic oil or fluid is, however, preferredin this embodiment as it reduces the requirements on the first andsecond piston high-pressure seals.

The first piston rod inner bore 76 can also include a set of stops, 86,88. Preferably retraction stops 86 are fixedly attached or formed of aprotrusion in a position to define the maximum upper travel limit of thesecond piston 92 and ensure secondary accumulator 98 integrity.Extension stops 88 can be formed within either the inner bore 76 of thepiston rod 72 or within or as part of the first piston aperture 74.FIGS. 3-5 depict the extension stop 88 as formed from part of a lowersurface 82 of the first piston 72 which effectively forms the aperture74 into a two-stage aperture within the first piston 72.

Peak loads or tension on the tensioner unit 28 occurring during aretraction and extension of the piston rod 32 are reduced due to theimplementation of floating piston 92 within the piston rod 32. While thefloating piston 92 is free-floating (not pegged against a retraction orextension stop, 86, 88), the primary accumulator pressure P1 equals thesame pressure P2 as in the secondary accumulator 98, and the totaleffective gas volume VgT of the unit is effectively increased by the gasvolume Vg2 of the secondary accumulator 98. The increase in gas volumeprovides a slower rate of increase in gas pressure P1 due to the addedvolume Vg2, and also reduces the peak load or tension of a givendownstroke (retraction of piston rod 32). This advantageously allows forthe use of a smaller tensioner unit 28. The net effect of the inclusionof the gas volume Vg2 into the total effective gas volume VgT and itsdeletion when pressure P1 decreases below the preselected pressure P2 isto limit the minimum tension applied to the supported system (FIG. 1) atfull extension and to also limit the maximum tension applied at nearfull retraction.

The following, for illustrative purposes only, is a description of theoperation of tensioner unit 28 on riser system of 20 according to anembodiment of the present invention as depicted in FIGS. 3-5. Inoperation, the hydro-pneumatic tensioner unit 28 maintains the risersystem 22 within a range of safe operating tensions as a work deck orplatform (not shown) moves relative to the upper portion of the risersystem 22. The hydro-pneumatic tensioner system 20 ideally should be ina similar configuration to that shown in FIG. 3. With thehydro-pneumatic tensioner system 20 properly installed, normally, thefirst piston 72 is established in the vicinity of the upper portion ofthe cylinder barrel 60, and the second piston 92 is normally establishedin a contact position with the lower stop 88. In this situation,pressure P1 on the lower side 96 of the second piston 92 is normallyless than the pressure P2 on the upper side 94 of the second piston 92.

In a partial retraction situation, as best shown in FIG. 4, normally,the first piston 72 is established in some mid-range position of thecylinder barrel 60, and the second piston 92 is still normallyestablished in a contact position with the lower stop 88. In thissituation, pressure P1 on the lower side 96 of the second piston 92 isstill normally less than the pressure P2 on the upper side 94 of thesecond piston 92 but with a decreased pressure differential (P2-P1)maintaining that position. The total effective gas volume VgT ismaintained substantially at that volume of the primary accumulator Vg1.As the piston 72 continues to stroke down (retract) tension increases ata sufficient rate to provide sufficient support to the riser system 22.

In a near full retraction situation, as best shown in FIG. 5, normally,in a situation where the first piston 72 was operating without thebenefit of the added second piston accumulator 98, tension wouldincrease rapidly. The second piston 92, however, having its own “charge”moves upward to maintain the two pressures P1 and P2 in substantialequilibrium unless the upper stop 86 is contacted. Thus, theimplementation of the second piston 92, within the piston assembly 70,results in the effective amount of gas VgT being effectively increasedover that of Vg1. The increase in gas volume provides a slower rate ofincrease in gas pressure P1 due to the added volume and also reduces thepeak load of a given downstroke (retraction of piston rod 32).

Beginning now with the piston assembly 70 in a fully retracted position(not shown), whereby the pressure in the primary accumulator P1 mayexceed that of the secondary accumulator 98 (piston 92 in contact withupper stop 86), as the piston 72 strokes up (extends), the pressure inthe primary accumulator P1 decreases until it substantially reaches thepressure of the secondary accumulator P2. At that point, the floatingpiston 92 becomes evenly loaded on either side (e.g. FIG. 5). Asmentioned above, while the floating piston 92 is free-floating, theprimary accumulator pressure P1 substantially equals that of thesecondary accumulator P2, which results in a total effective gas volumeVgT increase by the gas volume of the secondary accumulator Vg2.

Beginning at a near full retraction position, as best shown in FIG. 5,in an extension situation, without the second piston accumulator 98,tensions would decrease rapidly. The second piston 92, however, havingits own “charge” moves downward, maintaining the two pressures P1 and P2in substantial equilibrium until the lower stop 88 is contacted. Thus,as stated previously, due to the implementation of a second piston 92,within the piston assembly 70, the effective amount of gas VgT iseffectively increased. The increase in gas volume provides a slower rateof decrease in gas pressure due to the added volume. Once the primaryaccumulator pressure P1 decreases sufficiently, the second piston 92 isagain established in a contact position with the lower stop 88 (e.g.FIG. 4). The increase in stiffness is reduced at near full retractionunless pressure P1 increases above the charging pressure of thesecondary accumulator 98. The net effect of increasing the gas volumeVgT at the near full retraction positioned to decrease maximum stiffnessand to limit the minimum tension applied to the riser system 22 (FIG. 1)at full upstroke (extension of piston rod 32).

In another embodiment of the present invention, as perhaps best shown inFIG. 6, a tensioner unit 128 similar to the tensioner unit 28 describedwith respect to FIG. 3 can be modified to remove second piston 92 frombore 76 of piston rod 32. In this embodiment, the piston inner bore 176also interfaces and is in communication through its lower aperture 164with hydraulic fluid in the cylinder barrel 160. However, with secondpiston 92 removed, the pressure and any gas volume within a bore 176 ofpiston rod 132 is that of the primary accumulator 152. In thisembodiment, the gas volume contained within main body 130 and any gasvolume contained in piston rod 132 combine to form the primaryaccumulator gas volume Vg1. The amount of hydraulic fluid entering thepiston assembly inner bore 176 adjacent its lower side is directlydependent upon gas pressure P1 within the primary accumulator 152, whichis directly dependent upon stroke position of the piston rod 132. Note,although FIG. 6 depicts a gas volume in the bore 176 of piston rod 132,piston rod 132 may be filled entirely with part of the hydraulic volumeof the primary accumulator 152. Additionally, bore 176, at for exampleaperture 174, may be capped so that gas volume Vg1 is contained entirelywithin main body 130. Also note, if the structure described in FIG. 3were used to form this embodiment, optional features, such as upper andlower stops, 86, 88, described with respect to FIG. 3 are unnecessary.If installed, however, they do not affect the functional operation ofthe embodiment as configured and shown in FIG. 6.

In this embodiment, the tensioner unit 128 includes a secondaryaccumulator 121 having housing 101, separate from the cylinder barrel160, piston rod 132, and main body 130, which interfaces and is in fluidcommunication through its aperture 103 with primary accumulatorhydraulic fluid Vh1 (or gas Vg1) either in the cylinder barrel 160,piston rod 132, or main body 130 (as shown). Correspondingly, dependingon the configuration, either cylinder barrel 160, piston rod 132, ormain body 130, can have an aperture 105 and a fluid connection assembly107, as known and understood by those skilled in the art, to beconnected to the aperture 103 of the secondary accumulator housing 101.Preferably, the secondary accumulator housing 101 is in the form of anexternal cylinder having an inner bore 109. In the configuration shownin FIG. 6, the secondary accumulator housing 101 includes a fluidseparator in the form of a floating piston 111. The inner bore 109encloses the floating piston 111. The floating piston 111 has an upperand lower surface 113, 115, and a diameter that is substantially equalto the diameter of the secondary accumulator housing inner bore 109. Thefloating piston 111 can include pressure seals that slidingly engage theinner bore 109. The floating piston upper surface 113 in combinationwith the secondary accumulator housing inner bore 109 form an enclosure,which along with a gas volume Vg2 and a hydraulic volume Vh2, sealinglycontained on the upper side 113 of the floating piston 111, definesecondary accumulator 121 having a gas pressure P2. The lower side 115of the floating piston 111 in combination with the secondary accumulatorhousing inner bore 109 form an additional portion of the primaryaccumulator 152.

In the preferred configuration, the secondary accumulator housing innerbore 109 includes a set of stops 123, 125. Retraction stops 123 arefixedly attached and form a protrusion to define a maximum upper travellimit of the floating piston 111 and ensure secondary accumulator 121integrity. Extension stops 125 are formed within the secondaryaccumulator housing inner bore 109. Under normal conditions, wherepressure P2 of the secondary accumulator 121 is greater than thepressure P1 of the primary accumulator 152, the floating piston 111rests against extension stops 125. As with the embodiment depicted inFIGS. 3-5, the loads occurring during a retraction of the piston rod 32are reduced due to the implementation of the floating piston 111 withinthe secondary accumulator housing 101. As with the embodiment describedin FIG. 3, either the primary accumulator 152 or the secondaryaccumulator 121 may include charging connections (not shown) to permitintermittent or continuous charging of both the gas volumes Vg1, Vg2,and hydraulic volumes Vh1, Vh2, from an external pressure source (notshown). In another variation, the secondary accumulator 121 may includean interface (not shown) with another external accumulator (not shown)to provide added gas volume.

In another embodiment of the present invention, as perhaps best shown inFIG. 7, a tensioner unit 228 similar to the tensioner unit 128 describedwith respect to FIG. 6 can be modified to either cap lower end 164 ofcylinder barrel 160 and/or remove lower fluid aperture channel extension168, or, as shown in FIG. 7, cap channel extension 268 with cap 269. Themain body 230, in this embodiment, if implemented, generally functionsto provide protection and to provide an attachment point to a supportedstructure. As in the previous embodiment, described with respect to FIG.6, the piston inner bore 276 interfaces and is in communication throughits lower aperture 274 with hydraulic fluid in bore 266 of the cylinderbarrel 260. The amount of hydraulic fluid entering the piston assemblyinner bore 276 adjacent its lower side is directly dependent upon gaspressure P1 within the primary accumulator 252, which is directlydependent upon stroke position of piston 272 and piston rod 232. The gasvolume contained within the piston rod 232, however, in the depictedconfiguration, solely forms the primary accumulator gas volume Vg1.

In this embodiment, the tensioner unit 228 also includes secondaryaccumulator 121 having housing 101, separate from the cylinder barrel260, piston rod 232, and main body 230 (if so configured), whichinterfaces and is in fluid communication through its aperture 103 withprimary accumulator hydraulic fluid Vh1 in the cylinder barrel 260 (asshown). Correspondingly, cylinder barrel 260 has aperture 205 fluidlyconnected to fluid connection assembly 207 and to aperture 103 of thesecondary accumulator housing 101. As described with respect to theembodiment shown in FIG. 6, the secondary accumulator housing 101 alsoincludes a fluid separator in the form of a floating piston 111. Theinner bore 109 encloses the floating piston 111. The floating piston 111has an upper and lower surface 113, 115, and a diameter that issubstantially equal to the diameter of the secondary accumulator housinginner bore 109. The floating piston 111 can include pressure seals thatslidingly engage the inner bore 109. The floating piston upper surface113 in combination with the secondary accumulator housing inner bore 109form an enclosure, which along with a gas volume Vg2 and a hydraulicvolume Vh2, sealingly contained on the upper side 113 of the floatingpiston 111, define a secondary accumulator 121 having a gas pressure P2.The lower side 115 of the floating piston 111 in combination with thesecondary accumulator housing inner bore 109 form an additional portionof the primary accumulator 252.

In still another embodiment of the present invention, as perhaps bestshown in FIG. 8, a tensioner unit 328 similar to the tensioner unit 128described with respect to FIG. 6 can be modified to either remove innerbarrel 160 or to cap the lower end 164 of cylinder barrel 160 and removethe main body (tubular housing) 130 and channel extension 168. If theinner barrel 160 is removed, the diameter of the first piston 172 can bemodified so that the outer diameter of piston 372 is substantiallyequivalent to the inner diameter of the axial inner bore 358 of the mainbody or barrel 330. Correspondingly, back annulus 378 performs the samefunction described above and interfaces with the bore 358 of main body330 instead of bore 166 (FIG. 6) of cylinder barrel 160.

Regardless of the configuration selected, as with the previousembodiment described with respect to FIG. 7, the piston inner bore 376interfaces and is in communication through its lower aperture 374 withhydraulic fluid in the barrel housing the piston rod 332. The amount ofhydraulic fluid entering the piston assembly inner bore 376 adjacent itslower side is directly dependent upon gas pressure P1 within the primaryaccumulator 352, which is directly dependent upon stroke position ofpiston 372 and piston rod 332. The gas volume contained within thepiston rod 332, however, in the depicted configuration, again solelyforms the primary accumulator gas volume Vg1.

Still referring to FIG. 8, the tensioner unit 328 can include secondaryaccumulator 121 having housing 101, separate from either the main body(bore) 330 and piston rod 332, which interfaces and is in fluidcommunication through its aperture 103 with primary accumulatorhydraulic fluid Vh1 in the main body 330. Correspondingly, the main body330 has aperture 305 fluidly connected to a fluid connection assembly307 and to aperture 103 of the secondary accumulator housing 101. Thesecondary accumulator housing 101 also includes a fluid separator in theform of a floating piston 111, which along with secondary accumulatorhousing 101 forms an enclosure containing a gas volume Vg2 and hydraulicvolume Vh2, which define a secondary accumulator 121, as previouslydescribed, in detail, with respect to the embodiments shown in FIGS. 6and FIG. 7. Note, FIGS. 7 and 8 not only depict a possible position fora hydraulic Vh1 tap (aperture 305) but also a possible position of a gasVg1 tap in the piston rod 332 where the secondary accumulator housingand is configured in the form of an external gas accumulator (describedlater).

The previous embodiments, as shown in FIGS. 3-8 and described above,depict the housing for a secondary accumulator in the form of either apiston rod 32 having inner bore 76 (FIGS. 3-5) or secondary accumulatorhousing 101 having inner bore 109 and containing within a fluidseparator in the form of a floating piston 92, 111, sealingly engagingthe inner bore 76, 109, respectively (FIGS. 6-8). Regarding theembodiments described with respect to FIGS. 6-8, the floating piston 111sealingly and slidingly engages inner bore 109 of secondary accumulatorhousing 101 and functions in a similar manner as floating piston 92,described with respect to FIGS. 3-5 (previously described in detail).Alternative embodiments of the fluid separator in the secondaryaccumulator are, however, within the scope of the present invention.

For example, as best shown in FIG. 9, the floating piston 111 (FIGS.6-8) can be replaced by a bladder 211 that sealingly engages inner bore209 of secondary accumulator housing 201 in a fixed position. Similar tothe floating piston 111 implementation, upper surface 213 of the bladder211 in combination with the secondary accumulator housing inner bore 209form an enclosure, which along with a gas volume Vg2 sealingly containedon the upper side 213 of the bladder 211, define a secondary accumulator221 having a gas pressure P2. The lower side 215 of the bladder 211 incombination with the secondary accumulator housing inner bore 209 forman additional portion of the primary accumulator through aperture 203.

In operation, bladder 211 is selected to “balloon” when pressure P1 ofthe primary accumulator equals or exceeds pressure P2 of the secondaryaccumulator. Hydraulic volume Vh1 having pressure P1 serves to compressbladder 211 at a preselected pressure P2, which, in turn, serves tocompress the secondary accumulator gas volume Vg2 to substantially thesame extent as that of the hydraulic volume Vh1 having pressure P1compresses primary accumulator gas volume Vg1. This “ballooning effect”results in functionally combining the gas volume of the primaryaccumulator Vg1 and with the gas volume Vg2 of the secondary accumulatorwhen the pressure P1 of the primary accumulator equals or exceedspressure P2 of the secondary accumulator. This results in an increasedtotal gas volume VgT which serves to reduce maximum tension andstiffness during a downstroke of the piston rod. However, as with theconfiguration described with respect to FIG. 9, secondary hydraulicvolume Vh2 would be unnecessary. As stated above, the secondaryhydraulic volume Vh2 in the above described embodiments and theircorresponding configurations is generally used for the purpose ofmaintaining hydraulic seals of the floating piston. As the bladder 211is fixedly mounted, the second hydraulic fluid volume is unnecessary.Note also, the second piston 92 (FIGS. 3-5) can be replaced by a bladdersimilar to bladder 211 fixedly mounted within bore 76 of piston rod 32.

As best shown in FIG. 10, the floating piston 111 (FIGS. 6-8) can alsobe replaced by a fluid separator in the form of a valve or valvearrangement 311. In this embodiment of the present invention, valvearrangement 311, typically in the form of a pilot valve, has a firstfluid connection assembly 343 in fluid communication with pressurizedfluid in the primary accumulator through an aperture 105 (FIG. 6) and asecond fluid connection assembly 345 in fluid communication with thepressurized fluid contained within secondary accumulator housing 301through aperture 303 to intermittently connect pressurized fluid in theprimary accumulator with pressurized fluid in the secondary accumulatorhousing when pressure P1 in the primary accumulator pressure P1 equalsor exceeds the preselected pressure P2 of the secondary accumulator.Referring to the configurations shown in FIGS. 6-8, when combined withthe fluid separator shown in FIG. 10, the interface between the fluidseparator and the primary accumulator is with hydraulic volume Vh1. Thebore 309 of secondary accumulator housing 101 contains gas volume Vg2and hydraulic volume Vh2 which, in combination with the second fluidconnection assembly 345 of the valve arrangement 311, define a secondaryaccumulator 321.

In operation, as primary accumulator pressure P1 equals or exceedssecondary accumulator pressure P2, the valve arrangement 331 allows aportion of hydraulic volume Vh1 to enter the secondary accumulator andcompress the secondary accumulator gas volume Vg2 to substantially thesame extent the hydraulic volume Vh1 having pressure P1 compressesprimary accumulator gas volume Vg1. This “effect” results to combine thegas volume of the primary accumulator Vg1 and with the gas volume Vg2 ofthe secondary accumulator when the pressure P1 of the primaryaccumulator equals or exceeds pressure P2 of the secondary accumulator.This produces an increased total gas volume VgT which serves to reducemaximum tension and stiffness during a downstroke of the piston rod. Asthe pressure P1 of the primary accumulator decreases to that of thepreselected secondary accumulator pressure P2, the valve arrangement 311allows substantially the same amount of hydraulic fluid from the primaryaccumulator hydraulic volume Vh1 equivalent to the amount that enteredthe secondary accumulator hydraulic volume Vh2 to be returned throughthe valve arrangement 311 by the expanding secondary accumulator gasvolume Vg2. When the primary accumulator pressure P1 decreases below thesecondary accumulator preset pressure P2, the two hydraulic volumes Vh1,Vh2, are isolated from each other by valve arrangement 311 and the totalequivalent gas volume VgT of the tensioner unit is that of the primaryaccumulator gas volume Vg1.

As stated above, the previous embodiments shown in FIGS. 3-8 anddescribed above in detail, depict a housing for a secondary accumulatorcombined with a fluid separator either in the form of a piston, bladder,or valve arrangement, functionally engaged through use of a primaryaccumulator hydraulic volume Vh1. With respect to the embodimentsdescribed that utilize a separate external secondary accumulatorhousing, and with reference to FIGS. 6-8, little modification to thoseembodiments need be necessary in order to provide a secondaryaccumulator which is functionally engaged entirely through use of a gassuch as primary accumulator gas volume Vg1.

Referring to FIGS. 11-15, shown is the implementation of a tensionerunit 428, similar to that described with reference to FIG. 7, havingthree variations of a secondary accumulator in the form of the gasaccumulator. The tensioner unit 428 can have secondary accumulatorhousing 401 positioned in fluid (gas) communication with either of thepreviously described embodiments primary accumulator gas volume Vg1. Forexample, aperture 405 can be located in piston rod 432 to provideprimary accumulator pressure P1 to either of the embodiments of a fluidseparator, described above. Referring to the implementation of a fluidseparator taking the form of a floating piston 411 (FIGS. 11-13), thefloating piston 411 performs the same function as floating piston 111 asdescribed with respect to FIG. 6-8 or floating piston 92 described withrespect to FIGS. 3-5. In this configuration, however, the gas volumecontained within the bore 476 of piston rod 432, lower side 415 of thefloating piston 411 in combination with the secondary accumulatorhousing inner bore 409, and fluid connection assembly 407 positionedbetween apertures 403 and 405 combine to form the primary accumulatorhaving the primary accumulator gas volume Vg1. The bore 409 of thesecondary accumulator housing 401 in combination with the upper side 413of floating piston 411 and containing secondary accumulator gas volumeVg2 form the secondary accumulator 421. Note, no hydraulic volume Vh2 isused within the secondary accumulator 421 in this configuration. Notealso, due to the absence of hydraulic oil, the seals used to sealfloating piston 411 in inner bore 409 of the secondary accumulatorhousing 401 are gas seals, otherwise the configuration within secondaryaccumulator housing 401 can be substantially the same as that shown inFIGS. 6-8.

In operation, with a supported system such as the hydro-pneumatictensioner system 20 properly installed, normally, the first piston 472is established in the vicinity of the upper portion of the cylinderbarrel 460, and the floating piston 411 is normally established in acontact position with the lower stop 425. In this situation, pressure P1on the lower side 415 of the floating piston 411 is normally less thanthe pressure P2 on the upper side 413 of the floating piston 411.

In a partial retraction situation, as best shown in FIG. 12, normally,the first piston 472 is established in some mid-range position of thecylinder barrel 460, and the floating piston 411 is still normallyestablished in a contact position with the lower stop 425. In thissituation, pressure P1 on the lower side 415 of the floating piston 411is still normally less than the pressure P2 on the upper side 413 of thefloating piston 411 but with a decreased pressure differential (P2-P1)maintaining that position. The total effective gas volume VgT ismaintained substantially at that volume of the primary accumulator Vg1.As the piston 472 continues to stroke down (retract) tension increasesat a sufficient rate to provide sufficient support to the supportedsystem.

In a near full retraction situation, as best shown in FIG. 13, normally,in a situation where the first piston 472 was operating without thebenefit of the added second piston accumulator 421, tension wouldincrease rapidly. The floating piston 411, however, having its own“charge” moves upward to maintain the two pressures P1 and P2 insubstantial equilibrium unless the upper stop 423 is contacted. Thus,the implementation of the floating piston 411 in the external secondaryaccumulator housing 401 provides the same function as the second piston92 (FIGS. 3-5) within the piston rod 32. The implementation of thefloating piston results in the effective amount of gas VgT beingincreased over that of Vg1. The increase in gas volume provides a slowerrate of increase in gas pressure P1 due to the added volume and alsoreduces the peak load of a given downstroke (retraction of the pistonrod).

Referring now to FIG. 14, the floating piston 411 (FIGS. 11-13) can bereplaced by a bladder 511 that sealingly engages inner bore 509 ofsecondary accumulator housing 501 in a fixed position. In thisconfiguration, the gas volume contained within the bore 476 of pistonrod 432, lower side 515 of the bladder 511 in combination with thesecondary accumulator housing inner bore 509, and fluid connectionassembly 507 connected to aperture 503 combine to form a primaryaccumulator having the primary accumulator gas volume Vg1. As with thefloating piston 411, upper surface 513 of the bladder 511 in combinationwith the secondary accumulator housing inner bore 509 form an enclosure,which along with a gas volume Vg2 sealingly contained on the upper side513 of the bladder 511, define a secondary accumulator 521 having a gaspressure P2. Note, the second piston 92 (FIGS. 3-5) can also be replacedby a bladder similar to bladder 511 and fixedly mounted within bore 76of piston rod 32.

In operation, bladder 511 is selected to “balloon” when pressure P1 ofthe primary accumulator equals or exceeds pressure P2 of the secondaryaccumulator. However, in this configuration, gas volume Vg1, as opposedto hydraulic volume Vh1 having pressure P1 (FIG. 9), serves to compressbladder 511 at a preselected pressure P2, which, in turn, serves tocompress the secondary accumulator gas volume Vg2 to relatively the sameextent as that hydraulic volume Vh1 having pressure P1 compresses theportion of the primary accumulator gas volume Vg1 within bore 476 ofpiston rod 432. Where primary accumulator pressure P1 equals or exceedsthe secondary accumulator pressure P2, the gas volumes Vg1, Vg2,functionally combine to produce an increased total gas volume VgT whichserves to reduce maximum tension and stiffness during a downstroke ofthe piston rod 432. When the primary accumulator pressure P1 is againreduced below the preset secondary accumulator pressure P2, the totaleffective gas volume VgT returns to that of the primary accumulator gasvolume Vg1, and stiffnesses to the supported unit is increased.

As best shown in FIG. 15, the floating piston 411 (FIGS. 11-13) can alsobe replaced by a fluid separator in the form of an entirely gas fluidoperated valve or valve arrangement 611. In this configuration, valvearrangement 611, typically in the form of a pilot valve, has a firstfluid connection assembly 643 in fluid communication with pressurizedfluid in the primary accumulator through aperture 405 positioned toaccess primary accumulator gas volume Vg2, and a second fluid connectionassembly 645 in fluid communication with the pressurized fluid containedwithin secondary accumulator housing 601 through aperture 603 tointermittently connect pressurized fluid in the primary accumulator withpressurized fluid in the secondary accumulator housing when pressure P1in the primary accumulator pressure P1 equals or exceeds the preselectedpressure P2 of the secondary accumulator. Referring still to thetensioner unit 428 of FIG. 11 combined with the secondary accumulator621 shown in FIG. 15, when the interface between the fluid separator andthe primary accumulator is with gas volume Vg1, the bore 609 ofsecondary accumulator housing 601 contains gas volume Vg2 only which, incombination with the second fluid connection assembly 645 of the valvearrangement 611, defines the secondary accumulator 621.

In operation, as primary accumulator pressure P1 equals or exceedssecondary accumulator pressure P2, the valve arrangement 611 allows aportion of gas volume Vg1 to enter the secondary accumulator andcompress the secondary accumulator gas volume Vg2 to substantially thesame extent the gas volume Vg1 is compressed. This “effect” results tocombine the gas volume of the primary accumulator Vg1 and with the gasvolume Vg2 of the secondary accumulator when the pressure P1 of theprimary accumulator equals or exceeds pressure P2 of the secondaryaccumulator. This produces an increased total gas volume VgT whichserves to reduce maximum tension and stiffness during a downstroke ofthe piston rod 432. As the pressure P1 of the primary accumulatordecreases to that of the preselected secondary accumulator pressure P2,the valve arrangement 611 allows substantially the same amount of gasvolume that entered the secondary accumulator to be returned through thevalve arrangement 611 by the expanding secondary accumulator gas volumeVg2. When the primary accumulator pressure P1 decreases below thesecondary accumulator preset pressure P2, the two gas volumes Vg1, Vg2,are isolated from each other by valve arrangement 611 and the totalequivalent gas volume VgT of the tensioner unit is that of the primaryaccumulator gas volume Vg1.

Referring now to FIG. 16, an alternative embodiment, a tensioner unit528 similar to the tensioner unit 128 (FIG. 6) is modified to preventprimary accumulator hydraulic volume Vh1 or primary accumulator gasvolume Vg1 from entering either piston 572, piston rod 532, or both.This embodiment can interface with either of the secondary accumulatorsdescribed with respect to FIGS. 6-15. In the configuration shown in FIG.16, the primary accumulator includes the inner bore of the main body(barrel) 530 and the lower surface 582 of piston 572 along with thefluid connection assembly and the primary accumulator side of the fluidseparator, according to the type of secondary accumulator selected.

In a configuration where the tensioner unit 528 is implemented with agas-type secondary accumulator, such as those described with respect toFIGS. 11, 14, or 15, aperture 505 is positioned in the main body 530,above the maximum hydraulic volume fluid line (not shown) correspondingto the level of the primary accumulator hydraulic volume Vh1 within themain body 530 that is external to the cylinder barrel 560 and reachedwhen piston 572 is in full downstroke. Primary accumulator hydraulicvolume Vg1 interfaces with the fluid separator of the selected secondaryaccumulator to effectively combine the primary accumulator gas volumeVg1 with the selected secondary accumulator gas volume Vg2. In aconfiguration where the tensioner unit 528 is implemented with ahydraulic-type secondary accumulator, such as those described withrespect to FIGS. 6, 9, and 10, aperture 505 is positioned below theminimum hydraulic volume fluid line (not shown) corresponding to thelevel of the hydraulic volume Vh1 within the main body 530 when piston572 is in full up-stroke. The primary accumulator hydraulic volume Vh1interfaces with the fluid separator of the selected secondaryaccumulator to effectively combine the primary accumulator gas volumeVg1 with the selected secondary accumulator gas volume Vg2.

Referring now to FIG. 17, an alternative embodiment of the presentinvention includes a pull-type hydro-pneumatic tensioner unit 628 havinga primary accumulator housing 601 and a secondary accumulator housingincluding those described with respect to FIGS. 6, 9-11, 14, or 15. Apiston 672 similar to the one described in FIG. 16 moves axially withinmain body (barrel) 630 having inner bore 666. The piston 672 has apiston rod 632 extending from the lower side 682 of piston 672, thepiston rod extends through a piston rod aperture 662 and throughhigh-pressure seals associated with the aperture 662. The piston 672also includes high-pressure seals 690. The combination, as describedabove and as to be described below, forms an enclosure of varying sizedepending upon primary accumulator pressure P1 and stroke position ofthe piston rod 632.

In the preferred configuration, the diameter of the piston 672 issubstantially equivalent to the diameter of the inner bore 666 of mainbody 630, and the diameter of the piston rod 632 is preferablysubstantially equivalent to the inner diameter of the diameter of thepiston rod aperture 662. The volume formed between the upper side 684 ofthe piston 672 and the inner bore 666 of the main body 630 forms alow-pressure annulus that is preferably vented to the atmosphere. Thevolume formed between the lower side 682 of the piston 672 and the mainbody inner bore 666 forms a portion of the primary accumulator 652having a portion of the primary accumulator hydraulic volume Vh1. Themain body 630 also includes an aperture 605 located adjacent the lowerend of the main body to provide fluid communication between the mainbody portion of the primary accumulator and primary accumulator housing601. The primary accumulator housing 601 preferably takes the form of anexternal cylinder or barrel. The primary accumulator housing includesaperture 603 in fluid communication with main body 630 through a fluidconnection assembly 607 connected between aperture 603 of the primaryaccumulator housing 601 and aperture 605 of the main body 630. The bore609 of the primary accumulator housing 601 includes a portion of theprimary accumulator hydraulic volume Vh1. The bore 609 of the primaryaccumulator housing 601 also includes at least a portion of the primaryaccumulator gas volume Vg1, depending on the configuration of theselected secondary accumulator. The amount of primary accumulatorhydraulic volume Vh1 is dependent upon primary accumulator pressure P1and stroke position of piston 672.

The tensioner unit 628 according to this embodiment can at leastinterface with either of the secondary accumulators described withrespect to FIGS. 6-15. For illustrative purposes, the followingdiscussion will be with respect to the secondary accumulator 121described with respect to FIGS. 6-8. The primary accumulator housing 601and secondary accumulator housing 101 are in fluid communication througha fluid connection assembly 608 connected to aperture 602 of the primaryaccumulator housing 601 and aperture 103 of the secondary accumulatorhousing 101. In this configuration, the primary accumulator 652 includesthe inner bore of the main body (barrel) 630 and the lower surface 682of piston 672, fluid connection assembly 607, the bore 609 of theprimary accumulator housing 601, the fluid connection assembly 608, andthe bore 109 of secondary accumulator housing 101 in combination withthe primary accumulator side of the fluid separator according to thetype of secondary accumulator configuration selected. The secondaryaccumulator 121 includes the inner bore 109 of the secondary accumulatorhousing 101 and the secondary accumulator side of the fluid separatoraccording to the type of secondary accumulator configuration selected.

The configuration depicted in FIG. 17 includes a hydraulic-typesecondary accumulator similar to that described with respect to FIG. 6,whereby primary accumulator hydraulic volume Vh1 provides the impetus tofunctionally combine the primary accumulator gas volume Vg1 with thesecondary accumulator gas volume Vg2 to provide the tensioner unit 628an effective total gas volume VgT substantially equivalent to that ofthe sum of the primary accumulator gas volume Vg1 and secondaryaccumulator gas volume. If a gas-type accumulator is used, such as thatdescribed with respect to FIG. 14, the fluid connection assembly 608would instead be connected to the upper aperture 602 of primaryaccumulator housing 601 in communication with the primary accumulatorgas volume Vg1. Additionally, the maximum amount of primary accumulatorhydraulic volume Vh1 can be configured under expected extreme operatingconditions not to exceed a level within the bore 609 of the primaryaccumulator housing 601 to that of communicating with the upper aperture602. In this configuration, the primary accumulator gas volume Vg1provides the impetus to functionally combine the primary accumulator gasvolume Vg1 with the secondary accumulator gas volume Vg2 to provide thetensioner unit 628 an effective total gas volume VgT substantiallyequivalent to that of the sum of the primary accumulator gas volume Vg1and secondary accumulator gas volume Vg2.

The invention has several advantages. The hydro-pneumatic risertensioner units provide high nominal stiffness while limiting peak loadsat extreme extended and retracted positions. In one configuration, thetelescopic piston rod of the riser tensioner unit incorporates the boreor annulus of the piston rod as a secondary accumulator and incorporatesa floating piston within the annulus of the piston rod. In otherconfigurations, a separate housing having the bore or annulus is used toform the secondary accumulator. The total effective gas volume of theunit is increased by that of the secondary accumulator. During pistoncompression, the increased effective gas volume results in the load onthe riser system increasing at a much lower rate and a reduced peakload. During piston decompression, the increased total effective gasvolume results in a slower decrease in pressure and results in limitingthe minimum tension applied to the riser system. Correspondingly, asmaller primary accumulator may be utilized.

In the drawings and specification, there have been disclosed a typicalpreferred embodiment of the invention, and although specific terms areemployed, the terms are used in a descriptive sense only and not forpurposes of limitation. The invention has been described in considerabledetail with specific reference to these illustrated embodiments. It willbe apparent, however, that various modifications and changes can be madewithin the spirit and scope of the invention as described in theforegoing specification. For example, optionally, the system could beentirely pressurized by gas only, using a separate lubricant for theseals, as known by those skilled in the art. Also, if full retraction isnot required, the retraction stops used in the floating pistonconfigurations could be eliminated. Further, the accumulator could beconnected to the riser and the piston rod to the vessel in reverse towhat was described. Still further, the tensioner unit can be attached tothe top plate at an intermediate point along the barrel rather than atthe upper end as shown in figures. Correspondingly the tensioner unitcan be connected to the support module lower support frame at anintermediate point along the barrel rather than at the lower end asshown in the figures.

1. A tensioner unit, comprising: a barrel having a bore and an apertureon one end and having a pressurized fluid contained within and formingat least part of a primary accumulator, the primary accumulator having apreset volume of gas Vg1 at pressure P1; a first piston having a firstand second side and slidably carried in the bore of the barrel, thepiston having a piston rod that extends from the second side of thepiston and through the aperture of the barrel, having one of the sidesof the piston in communication with the pressurized fluid, andpositioned to increase the pressure P1 of the primary accumulator whenthe piston strokes in the direction of the pressurized fluid; asecondary accumulator housing having a bore and forming at least part ofa secondary accumulator, the secondary accumulator having a presetvolume of gas Vg2 at preselected pressure P2; a fluid separator havingfirst and second sides and positioned between the primary and secondaryaccumulators to maintain functional separation of fluid volumes of theprimary and secondary accumulators when the primary accumulator pressureP1 is less than the secondary accumulator pressure P2, and to allowfunctional combining of the fluid volumes of the primary and secondaryaccumulators when the primary accumulator pressure P1 is greater than orequal to the secondary accumulator pressure P2; wherein an effectivetotal gas volume VgT available to the tensioner to maintain tension on asupported riser system substantially equals the primary accumulator gasvolume Vg1 when the primary accumulator pressure P1 is less than thesecondary accumulator pressure P2; and wherein the effective total gasvolume VgT available to the tensioner to maintain tension on thesupported system substantially equals the sum of the primary accumulatorgas volume Vg1 plus the secondary accumulator gas volume Vg2 when thepressure P1 is greater than or equal to the secondary accumulatorpressure P2.
 2. The tensioner unit of claim 1, further comprising atubular housing surrounding the barrel and having pressurized fluidcontained within and forming a part of the primary accumulator, andwherein: the piston rod includes a bore which forms the secondaryaccumulator housing; the barrel has a port on one end in fluidcommunication with the tubular housing; the first piston has an openingin fluid communication with the port; the bore of the piston rod is influid communication with the opening in the piston; the fluid separatorcomprises a second piston having first and second sides and sealinglyand slidably carried in the bore of the piston rod; the bore of thebarrel, the bore of the tubular housing, and the first sides of thefirst and second piston define the primary accumulator; and the bore ofthe piston rod and the second side of the second piston define thesecondary accumulator.
 3. The tensioner unit of claim 1, wherein: thefirst piston has an opening in fluid communication with the barrel; thepiston rod includes a bore in fluid communication with the opening inthe piston; the secondary accumulator housing is separate from thebarrel and is in fluid communication with the second side of the fluidseparator; and the first side of the fluid separator is in fluidcommunication with an opening in one of the bore of the piston rod andthe bore of the barrel.
 4. The tensioner unit of claim 3, wherein: thebore of the barrel, the bore of the piston rod, the bore of thesecondary accumulator housing, the first side of the first piston, andfirst side of the fluid separator define the primary accumulator; thebore of the secondary accumulator housing and a second side of the fluidseparator define the secondary accumulator; the fluid separatorcomprises a second piston sealingly and slidably carried in the bore ofthe secondary accumulator housing; and the bore of the secondaryaccumulator housing further includes an extension stop spaced from anend of the secondary accumulator housing which limits maximum travel ofthe second piston during piston rod extension corresponding to adecrease in primary accumulator pressure P1 below that of the secondaryaccumulator pressure P2.
 5. The tensioner unit of claim 3, wherein: thebore of the barrel, the bore of the piston rod, the bore of thesecondary accumulator housing, the first side of the first piston, andfirst side of the fluid separator define the primary accumulator; thebore of the secondary accumulator housing and a second side of the fluidseparator define the secondary accumulator; and the fluid separatorcomprises a bladder sealingly engaged in a fixed position within thebore of the secondary accumulator housing.
 6. The tensioner unit ofclaim 3, wherein: the fluid separator comprises a valve having a firstfluid connection assembly in fluid communication with the pressurizedfluid in one of the bore of the piston rod and the bore of the barrel,and a second fluid connection assembly in fluid communication with thepressurized fluid in the bore of the secondary accumulator housing; thebore of the barrel, the bore of the piston rod, the first side of thefirst piston, and the first fluid connection assembly of the valvedefine the primary accumulator; the bore of the tubular housing and thesecond fluid connection assembly of the valve define a secondaryaccumulator; and wherein the valve connects the pressurized fluid in thebore of the barrel with the pressurized fluid in the bore of thesecondary accumulator housing when the pressure P1 in the primaryaccumulator exceeds the preselected pressure P2 of the secondaryaccumulator.
 7. The tensioner unit of claim 1, further comprising atubular housing surrounding the barrel, the tubular housing having abore, a fluid communication opening, and pressurized fluid containedwithin, and forming a part of the primary accumulator, and wherein: thesecondary accumulator housing is separate from both the barrel and thetubular housing and is in fluid communication with the second side ofthe fluid separator; the first side of the fluid separator is in fluidcommunication with the tubular housing through the fluid communicationopening; and the first side of the first piston forms a fluid barrier.8. The tensioner unit of claim 7, wherein: the bore of the barrel, thebore of the tubular housing, the bore of the secondary accumulatorhousing, the first side of the first piston, and first side of the fluidseparator define the primary accumulator; the bore of the secondaryaccumulator housing and a second side of the fluid separator define thesecondary accumulator; the fluid separator comprises a second pistonsealingly and slidably carried in the bore of the secondary accumulatorhousing; and the bore of the secondary accumulator housing furtherincludes an extension stop spaced from an end of the secondaryaccumulator housing which limits maximum travel of the second pistonduring piston rod extension corresponding to a decrease in primaryaccumulator pressure P1 below that of the secondary accumulator pressureP2.
 9. The tensioner unit of claim 7, wherein: the bore of the barrel,the bore of the piston rod, the first side of the first piston, the boreof the secondary accumulator housing, and first side of the fluidseparator define the primary accumulator; the bore of the secondaryaccumulator housing and a second side of the fluid separator define thesecondary accumulator; and the fluid separator comprises a bladdersealingly engaged in a fixed position within the bore of the secondaryaccumulator housing.
 10. The tensioner unit of claim 7, wherein: thefluid separator comprises a valve having a first fluid connectionassembly in fluid communication with the pressurized fluid in the boreof the tubular housing, and a second fluid connection assembly in fluidcommunication with the pressurized fluid in the bore of the secondaryaccumulator housing; the bore of the barrel, the first side of the firstpiston, the bore of the tubular housing, and the first fluid connectionassembly of the valve define the primary accumulator; the bore of thesecondary accumulator housing and the second fluid connection assemblyof the valve define the secondary accumulator; and wherein the valveconnects the pressurized fluid in the bore of the tubular housing withpressurized fluid in the bore of the secondary accumulator housing whenthe pressure P1 in the primary accumulator exceeds the preselectedpressure P2 of the secondary accumulator.
 11. The tensioner unit ofclaim 1, further comprising a primary accumulator housing having a bore,a plurality of fluid communication apertures, and having pressurizedfluid contained within and forming a part of the primary accumulator,and wherein: the first side of the first piston forms a fluid barrier;the primary accumulator housing is separate from the barrel and is influid communication with the barrel and the first side of the fluidseparator through the fluid communication apertures; the secondaryaccumulator housing is separate from the barrel and the primaryaccumulator housing and is in fluid communication with the second sideof the fluid separator; and the first side of the fluid separator is influid communication with the bore of the primary accumulator housing andbore of the barrel through the fluid communication apertures.
 12. Thetensioner unit of claim 11, wherein: the bore of the barrel, the firstside of the first piston, the bore of the primary accumulator housing,the bore of the secondary accumulator housing, and the first side of thefluid separator define the primary accumulator; the bore of thesecondary accumulator housing and the second side of the fluid separatordefine the secondary accumulator; the fluid separator comprises a secondpiston sealingly and slidably carried in the bore of the secondaryaccumulator housing; and the bore of the secondary accumulator housingfurther includes an extension stop spaced from an end of the secondaryaccumulator housing which limits maximum travel of the second pistonduring piston rod extension corresponding to a decrease in primaryaccumulator pressure P1 below that of the secondary accumulator pressureP2.
 13. The tensioner unit of claim 11, wherein: the bore of the barrel,the first side of the first piston, the bore of the primary accumulatorhousing, the bore of the secondary accumulator housing, and the firstside of the fluid separator define the primary accumulator; the bore ofthe secondary accumulator housing and the second side of the fluidseparator define the secondary accumulator; and the fluid separatorcomprises a bladder sealingly engaged in a fixed position within thebore of the secondary accumulator housing.
 14. The tensioner unit ofclaim 11, wherein: the fluid separator comprises a valve having a firstfluid connection assembly in fluid communication with the pressurizedfluid in the bore of the primary accumulator housing which is incommunication with the bore of the barrel, and a second fluid connectionassembly in fluid communication with the pressurized fluid in the boreof the secondary accumulator housing; the bore of the barrel, the firstside of the first piston, the bore of the primary accumulator housing,and the first fluid connection assembly of the valve arrangement definethe primary accumulator; the bore of the secondary accumulator housingand the second side of the fluid separator define the secondaryaccumulator; and the valve connects the pressurized fluid in the bore ofthe primary accumulator housing and the bore of the barrel with thepressurized fluid in the bore of the secondary accumulator housing whenthe pressure P1 in the primary accumulator exceeds the preselectedpressure P2 of the secondary accumulator.
 15. A riser tensioning systemhaving a riser extending between subsea well equipment and a floatingvessel having an operational platform engaged with the riser, atensioner unit comprising: a barrel having a port on one end; a firstpiston slidingly carried in the barrel and having an opening in fluidcommunication with the port; a piston rod extending from the barrel andhaving a bore in fluid communication with the opening in the piston; asecond piston sealingly and slidably carried in the bore of the pistonrod; a primary accumulator in fluid communication with the port andhaving pressurized fluid therein that communicates with first sides ofthe first and second piston; and wherein: the bore of the piston rod anda second side of the second piston define a secondary accumulatorcontaining a pressurized fluid; the tensioner unit is positioned betweenthe riser and the platform; a first section of the tensioner unit isconnected to the riser; and a second section of the tensioner unit isconnected to the platform.
 16. The tensioning system of claim 15,wherein the bore of the piston rod of the tensioner unit includes anextension stop which limits maximum travel of the second piston duringpiston rod extension.
 17. The tensioning system of claim 15, wherein thebore of the piston rod of the tensioner unit includes a retraction stopspaced from an end of the piston rod which limits maximum travel of thesecond piston during piston rod retraction.
 18. The tensioning system ofclaim 15, wherein the primary accumulator of the tensioner unitcomprises a tubular housing surrounding the barrel.
 19. The tensioningsystem of claim 15, wherein the tensioner unit further comprises abarrel extension extending from the barrel which provides for fluidcommunication between the barrel port and the primary accumulator.
 20. Atensioner unit, comprising: a barrel including a bore; a first pistonslidingly carried in the bore of the barrel and having an opening influid communication with a pressurized fluid; a piston rod extendingfrom the barrel and having a bore in fluid communication with theopening in the piston; a secondary accumulator housing having a bore; afluid separator sealingly engaging the bore of the secondary accumulatorhousing and having a first and a second side to separate a plurality ofvolumes of fluid; a primary accumulator in fluid communication with thebore of the secondary accumulator housing through an opening in at leastone of the piston rod and barrel and having pressurized fluid thereinthat communicates with the first side of the first piston and first sideof the fluid separator; and the bore of the secondary accumulatorhousing and a second side of the fluid separator defining a secondaryaccumulator containing a pressurized fluid within.
 21. The tensionerunit of claim 20, wherein the fluid separator comprises a second pistonsealingly and slidably carried in the bore of the secondary accumulatorhousing.
 22. The tensioner unit of claim 21, wherein the bore of thesecondary accumulator housing further includes an extension stop spacedfrom an end of the secondary accumulator housing which limits maximumtravel of the second piston during piston rod extension corresponding toa decrease in primary accumulator pressure below that of the secondaryaccumulator pressure.
 23. The tensioner unit of claim 20, wherein thefluid separator comprises a bladder sealingly engaged in a fixedposition within the bore of the secondary accumulator housing.
 24. Atensioner unit, comprising: a barrel having a port on one end and incommunication with a pressurized fluid; a tubular housing surroundingthe barrel and having a bore, an opening, and an aperture on one end; apiston having a first and second side, slidingly carried in the bore ofthe barrel, and having a piston rod extending from the barrel on thesecond side of the piston through the aperture; a secondary accumulatorhousing including a bore, an opening in the bore in fluid communicationwith the opening in the bore of the tubular housing, and a fluidseparator sealingly engaging the bore of the secondary accumulatorhousing and having a first and a second side to separate a plurality ofvolumes of fluid; a primary accumulator in fluid communication with theport of the barrel and the opening in the bore of the tubular housing,and having pressurized fluid therein that communicates with the firstside of the piston and the first side of the fluid separator; and thebore of the secondary accumulator housing and the second side of thefluid separator defining a secondary accumulator containing apressurized fluid.
 25. The tensioner unit of claim 24, wherein the fluidseparator comprises a second piston sealingly and slidably carried inthe bore of the secondary accumulator housing.
 26. The tensioner unit ofclaim 25, wherein the bore of the secondary accumulator housing furtherincludes an extension stop spaced from an end of the secondaryaccumulator housing which limits maximum travel of the second pistonduring piston rod extension corresponding to a decrease in primaryaccumulator pressure below that of the secondary accumulator pressure.27. The tensioner unit of claim 24, wherein the fluid separatorcomprises a bladder sealingly engaged in a fixed position within thebore of the secondary accumulator housing.
 28. A tensioner unit,comprising: a barrel having a bore, a fluid communication opening, anaperture on one end, and having a volume of pressurized fluid therein; afirst piston having a first and a second side, slidingly carried in thebore of the barrel and having a piston rod that extends from the pistonthrough the fluid communication opening of the barrel; a primaryaccumulator housing having a bore, a plurality of fluid communicationopenings, and in fluid communication with the fluid communicationopening of the barrel; a secondary accumulator housing having a bore, afluid communication opening, and having a fluid separator sealinglyengaging the bore of the secondary accumulator housing, the fluidseparator having a first and a second side to separate fluid in thesecondary accumulator housing, the secondary accumulator housing furtherbeing in fluid communication with the primary accumulator housingthrough the fluid communication opening of the secondary accumulatorhousing and one of the fluid communication openings of the primaryaccumulator housing; a primary accumulator in fluid communication withthe fluid communication opening of the barrel and the fluidcommunication opening of the secondary accumulator housing and havingpressurized fluid that communicates with the first side of the firstpiston and first side of the fluid separator; and the bore of thesecondary accumulator housing and the second side of the fluid separatordefining a secondary accumulator containing a pressurized fluid.
 29. Thetensioner unit of claim 28, wherein the fluid separator comprises asecond piston sealingly and slidably carried in the bore of thesecondary accumulator housing.
 30. The tensioner unit of claim 29,wherein the bore of the secondary accumulator housing further includes alower stop spaced from an end of the secondary accumulator housing whichlimits maximum travel of the second piston during piston rod extensioncorresponding to a decrease in pressure of the primary accumulator belowthe pressure of the secondary accumulator.
 31. The tensioner unit ofclaim 29, wherein the fluid separator comprises a bladder sealinglyengaged in a fixed position within the bore of the secondary accumulatorhousing.
 32. A tensioner unit, comprising: a barrel including a bore andan aperture on one end and having a pressurized fluid contained withinand forming at least part of a primary accumulator; a piston incommunication with a pressurized fluid and slidingly carried in the boreof the barrel and having a piston rod that extends from the pistonthrough the aperture; a secondary accumulator housing having a bore anda pressurized fluid having a preselected pressure contained within; avalve arrangement having a first fluid connection assembly in fluidcommunication with the pressurized fluid in one of the bore of thepiston rod and the bore of the barrel, and a second fluid connectionassembly in fluid communication with the pressurized fluid in the boreof the secondary accumulator housing to connect the pressurized fluid inthe bore of the barrel with the pressurized fluid in the bore of thesecondary accumulator housing when the pressure of the pressurized fluidin the bore of the barrel exceeds the preselected pressure of thepressurized fluid in the bore of the secondary accumulator housing; thebore of the barrel, and the first fluid connection assembly of the valvearrangement defining a primary accumulator; and the bore of thesecondary accumulator housing and the second fluid connection assemblyof the valve arrangement defining a secondary accumulator.
 33. A methodof maintaining a selected range of tension on a riser extending betweensubsea well equipment and a floating vessel, comprising the steps of:providing a barrel having a bore and an aperture on one end, the barrelforming at least part of a primary accumulator; mounting a pistonslidably in the barrel, the piston having a piston rod extending fromthe barrel through the aperture; providing a secondary accumulatorhousing having a bore and forming at least part of a secondaryaccumulator; mounting a fluid separator between the primary andsecondary accumulators; connecting either the piston rod or the barrelto the vessel and the other to the riser; applying fluid pressure fromthe primary accumulator to a first side of the first piston and a firstside of the fluid separator; applying fluid pressure into the secondaryaccumulator housing on a second side of the fluid separator untilpressure on both sides of the fluid separator equal; applying tension tothe riser by the force due to pressure of fluid in the primaryaccumulator, urging the piston rod to extend; if vessel moves closer tothe subsea equipment, allowing the piston rod to extend farther andallowing the fluid separator to functionally isolate the fluid volume inthe secondary accumulator from the fluid volume in the primaryaccumulator to maintain tension range; and if vessel moves farther fromthe subsea equipment, allowing the piston rod to retract and allowingthe fluid separator to functionally combine the fluid volume in thesecondary accumulator with the fluid volume in the primary accumulatorto maintain tension range.
 34. The method of claim 33, furthercomprising limiting maximum travel of the second piston during pistonrod extension by contacting the second piston with an extension stop.35. The method of claim 33, further comprising limiting maximum travelof the second piston during piston rod retraction by contacting thesecond piston with a retraction stop.